1. Field of the Invention
The present invention relates to a rate control method whereby a system for providing a multimedia service across a network, such as video on demand (hereinafter referred to as a VOD), transmits to the network multimedia data, such as video/audio data, at a requested speed.
2. Related Arts
Because of the standardization of digital audio/video coding systems, such as ISO/MPEG (Moving Picture Expert Group)-2, and the expansion of networks using B-ISDN and ATM (Asynchronous Mode Transfer)-LAN, VOD service and various other bidirectional multimedia services can be realized.
VOD is a system that is constituted by a server (video server) for storing in a cumulative medium, such as a hard disk, a larger quantity of video data that are encoded in advance, and for distributing video data to a client upon the client's request, a client for displaying contents of a request for information issued by a subscriber and video data that are supplied; and a wide network (an ATM network) that can cope with a transfer and an exchange of video data.
FIG. 11 is a diagram illustrating the general configuration of a VOD service system. A video server 100 and a client 200 are connected to an ATM network 300, and upon receipt of a request from the client, MPEG2 data is transmitted by the server 100.
The video server 100 includes a storage device 101, such as a hard disk; a data transfer section 102; and a controller 103 for controlling these components. The client 200 includes a data receiver 210; an MPEG decoder 202; and a controller 203 for controlling these components.
A mapping method (MPEG over ATM) for mapping MPEG data over an ATM cell, which is used to transfer MPEG2 data, has been standardized by an ATM forum. The contents of the mapping are shown in FIGS. 12A, 12B and 12C. This standard will also be adopted by DAVIC, which is an organization established for the standardization of VOD systems.
Every two TS packets AA (188 bytes) (see FIG. 12A) of MPEG2 data are defined as a packet AAL5 (CPCS-PDU), and a trailer BB of eight bytes is added to the packet AAL5 to define a CPCS-PDU (AAL5 packet) (see FIG. 12B). Thereafter, the CPCS-PDU is disassembled to form eight ATM cells for transmission to a network. It should be noted that an ATM header DD of five bytes is provided for each ATM cell (see FIG. 12C).
To realize the video server 100, the MPEG data must be transmitted in the format shown in FIG. 12 that conforms to the system determined by the MPEG over ATM method. Another important function is control of the rate at which the ATM cells are transmitted to the network. This is the control for maintaining the speed at which the video contents are coded for transmission of the ATM cells to the network.
When this control is not performed precisely (clock precision: on an order of several tens of ppm, if available), a one-hour program, for example, will be ended within 59 minutes.
Taking the function of the video server 100 into account, since a conversion process between the AAL/ATM layers in FIGS. 12A through 12C is performed and an ATM cell is transmitted, a transmission queue is so controlled that it is never empty in order to maintain communication with a fixed rate.
A problem encountered here is that the value for a rate that can be set for the AAL/ATM layer conversion is a discrete value. That is, since cell transmission timing is generated by counting source clocks that are input for such control, the rate value can not help but be a discrete value.
In general, when the transfer speed does not match the data coding speed, stuff data (a packet) is inserted to absorb a difference in the speeds. An example structure is shown in FIG. 13. In FIG. 13, a buffer circuit 50, a multiplexer 51 and a stuff packet generator 52 are provided in the structure.
The buffer circuit 50 reads and fetches coded data in consonance with a coding clock timing, and transmits that data in consonance with a transmission clock timing. The stuff packet generator 52 detects an underflow for the data accumulated in the buffer circuit 50 and transmits stuff data (a packet) to the multiplexer 51 for insertion in the data.
If the same structure is to be applied for the coded data sent from the video server 100, however, a clock for coding can not be reproduced because the coded data from the video server 100 is stored in the storage device 101. Therefore, the configuration in FIG. 13 can not be applied.
In order to match the MPEG coding rate for information contents and the transmission rate for ATM cells, either (a) the information contents are coded at a coding rate that corresponds to the ATM cell transmission rate, or (b) an analog adjustment of an oscillator is performed that provides for the transmission rate a reference frequency that is in consonance with the coding speed.
With the remedial process (a), however, various vendors can not cope with the data contents, and the flexibility of the system will be lost. With the remedial process (b), since the coding rates for a number of different types of data contents are supported, a plurality of ATM interface sections, on which are mounted oscillators having different frequencies, must be provided for the individual transmission rates, with the result that there will be an increase in the system costs.